Sheet-fed printing press with screen-printing cylinder

ABSTRACT

A printing unit of a sheet-fed printing press includes two cooperating screen-printing cylinders which are in direct contact. These two screen-printing cylinders define a printing gap through which the sheets to be printed pass during their printing step.

FIELD OF THE INVENTION

The present invention is directed to a printing unit with ascreen-printing cylinder. The screen printing cylinder has a movabledoctor blade device. Two directly cooperating screen-printing cylinders,each with at least one working doctor blade, may also be provided.

BACKGROUND OF THE INVENTION

EP 07 23 864 B1 has disclosed a printing unit for a rotary printingpress. A first screen-printing cylinder, together with a secondcylinder, forms a printing nip, in which a print stock is printed.

This known printing unit is only suitable for one-sided printing.

DE 26 38 344 A1 discloses two cooperating screen-printing cylinders.

JP 63-071350 A discloses two opposing printing cylinders, each with adoctor blade. A separation between a doctor blade and a counter-pressuredevice is not provided.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a printing unit with ascreen-printing cylinder.

The object is attained according to the invention by the provision of atleast one screen-printing cylinder with a movable doctor blade. Twodirectly cooperating screen-printing cylinders can also be used. Eachhas a working doctor blade and a counter-pressure device. Thecounter-pressure device of one of the screen-printing cylinders supportsthe working doctor blade of the other screen-printing cylinder.

The advantages that can be achieved with the present invention arecomprised particularly in the fact that the printing press permits firstforme printing and second forme printing in the screen-printing processwith matching registers to be executed in a single printing procedure.

To this end, the present invention provides that the second cylinder isalso a screen-printing cylinder so that each screen-printing cylinder isused to print a respective side of a print stock being fed through theprinting nip which is defined by the two screen-printing cylinders.

In order to permit an exact, register-matching printing, even at theedge of a sheet-like print stock, preferably at least one of the twoscreen-printing cylinders is equipped with a sheet gripping mechanismfor the sheet-like print stock.

In order for this sheet gripping mechanism to be able to pass throughthe printing nip, it is necessary for at least one of the twoscreen-printing cylinders to have an indentation on its circumferencesurface. A doctor blade device disposed inside the screen-printingcylinder can preferably be moved radially so that it can move out of theway of the indentation.

It is also preferable for the doctor blade devices of the twoscreen-printing cylinders to each have a working doctor blade, each ofwhich working doctor blades which touches the interior of the screen ofits respective screen-printing cylinder when pressing ink through it,the respective working doctor blades of the two doctor blade devicesbeing exactly aligned with each other, so that the working doctor bladeof each doctor blade device compensates for a pressure exerted on itsscreen by the working doctor blade of the other. This arrangementprovides the simple assurance that there is a sufficient pressure in theprinting nip to press the ink required for the printing through thescreens and to transfer it onto the print stock. On the other hand, anundesirable deformation of each screen by the pressure of the doctorblade device associated with it is prevented because the respectiveother doctor blade device exerts a corresponding counter-pressure.

Alternatively to this, each doctor blade device can also have acounter-pressure device spaced apart from its associated working doctorblade in the circumference direction of the screen-printing cylinder.Each such counter-pressure device is aligned with the respective workingdoctor blade of the other doctor blade device and compensates for thepressure which this other doctor blade device exerts.

In addition to their previously well-known and customary task ofapplying ink to the screen-printing cylinder, the doctor blade devicesthus also perform the task of the counter-pressure cylinder that isusually provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are shown in the drawingsand will be described in detail below.

FIG. 1 is a schematic representation of a printing press with a printingunit in accordance with the present invention;

FIG. 2 shows a schematic section through the printing unit of themachine of FIG. 1;

FIGS. 3a and 3 b respectively show the printing nip and partial regionsof the screen-printing cylinders that form the printing nip, inaccordance with a first preferred embodiment of the printing unit of thepresent invention, in two phases of its rotary motion;

FIGS. 4 and 5 show two configurations of an indented section of thecircumference surface of the screen-printing cylinder; and

FIG. 6 shows the printing nip and partial regions of the screen-printingcylinders that form the printing nip, in accordance witha secondpreferred embodiment of the printing unit of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, there is shown a schematic view, e.g. ofa sheet-fed rotary printing press in which a printing unit 4, inaccordance with the present invention, is used. The printing press has asheet feeder 1 with a sheet stacker 2, from whose top, which isautomatically kept at a constant height, sheets to be printed are fedindividually or in a continuous stream by a belt conveyor 3 to theprinting unit 4. The printing unit 4 takes the sheets, one at a time,prints them, and outputs them to a second chain conveyor 6, which, inthe case of multicolor printing, feeds them to other printing units likethe printing unit 4 or, as shown here, feeds them directly to an outputstack 5.

The sheets pass through the printing unit 4 from top to bottom. Theprinting unit can be seen more clearly in FIG. 2 which gives a largerscale depiction of its structure.

Two transport cylinders 7, the upper of which is shown only partially inFIG. 2, are each provided with sheet gripping mechanisms, each in arespective section 8 of their circumferences, in order to grip theleading edges of sheets, which are to be printed, from the first beltconveyor 3 in a register-matching manner. The lower of the two transportcylinders 7 rolls in contact with a first screen-printing cylinder 12,which is likewise equipped with a sheet leading edge gripping mechanism29 in a section 11 of its circumference. In particular, this sheetleading edge gripping mechanism 29 may be a rotatable shaft with sheetgripper fingers, as seen in FIG. 3a and which is adapted for taking thesheets from the lower transport cylinder 7. The first screen-printingcylinder 12, together with a second screen-printing cylinder 17, forms aprinting nip 14 through which the sheets held by the sheet leading edgegripper 29 of the first screen-printing cylinder 12 are conveyed. Thetwo screen-printing cylinders 12 and 17 can be rotated in unison in sucha way that with each pass through the printing nip 14, the sheet gripper29 of the first screen-printing cylinder 12 coincides with, or isaligned with, a channel-like indented section 18 of the secondscreen-printing cylinder 17.

The operation of the screen-printing cylinders 12 and 17 and a firstpreferred embodiment of their construction will be described below, andtaken in conjunction with FIGS. 3a and 3 b.

FIGS. 3a and 3 b each show a view of a partial section of the twocooperating and coacting screen-printing cylinders 12 and 17 in thevicinity of the printing nip 14.

At each of its axial ends, each screen-printing cylinder 12 and 17 has asupport ring 22 whose outer circumference has a screen 23 stretched ontoit. Each screen 23 is preferably made of silk or polyamide gauze orbronze wire mesh. On the interior of each screen-printing cylinder 12and 17, a doctor blade device 24 is provided, whose position in theradial direction of its associated screen-printing cylinder iscontrolled by a curved body, in this instance a guide slot 26 situatedat the ends of the screen-printing cylinder 12 or 17, and through whichslot 26 a cylindrical guide projection 27 of the doctor blade device 24extends. Outside the screen-printing cylinder 12 or 17, the guideprojection 27 is supported at both ends so that it can move in thedirection of a line 28 extending between and connecting the rotationaxes of the screen-printing cylinder 12 and the screen-printing cylinder17. FIG. 3a shows the doctor blade device 24 in a position in which thesheet gripper 29 of the screen-printing cylinder 12 is passing throughthe printing nip 14 between the two cylinders. In the vicinity of thesheet gripper 29 and opposite from it, the screen 23 has a screensection 31 that is indented radially inward. The guide slot 26 has anarc-shaped guide slot section 33, which is not shown completely in FIG.3a, and which is concentric to the cylindrical outer surface of thescreen 23, and an inwardly indented guide slot section 32, whosecurvature corresponds to that of the screen section 31. The curvature ofthe guide slot section 32 is selected so that when the guide slotsection 32 moves past the guide projection 27 during the rotation of thescreen-printing cylinder 12, the doctor blade device 24 is retractedradially inward so far that it exerts only a minimal pressure againstthe screen 23, which minimal pressure produces no appreciabledeformation of the screen 23 in the screen section 31, or the doctorblade device 24 is retracted so far that it loses all contact with thescreen 23 and consequently exerts no pressure on its screen section 31,which pressure could otherwise deform this screen section 31 and damageit during the course of operation.

FIG. 3b shows the position of the doctor blade devices 24 after theguide slot section 32 has passed the guide projection 27. The arc-shapedsection 33 of the guide slot 26 keeps the doctor blade devices 24pressed against the inside of the screen 23 so that an ink 34 disposedagainst a working doctor blade 19 of the doctor blade device 24 ispressed through the open regions of the screen 23 and is thus applied toa sheet of print stock conveyed through the printing nip 14 defined bythe two screen-printing cylinders 12 and 17.

In this phase of the rotation of the two screen-printing cylinders 12and 17, the respective lips 16 of the two working doctor blades 19touching the screens 23 are oriented toward each other and rest exactlyin a plane defined by the axes of the two screen-printing cylinders 12and 17, which plane is represented by the line 28 in FIG. 3b

The parallelism of the lips 16 is of great importance for satisfactoryprinting results. If the lips 16 are not parallel to each other, butrather intersect at an angle, there is the possibility that away fromthe intersecting point of the two respective doctor blade lips 16, thescreen 23 will move away from the pressure exerted against its inside bythe doctor blade device 24, with the result that little or no ink 34 ispushed through the screen 23, and this ink, for lack of effectivepressure, is transferred either incompletely or not at all to a sheetbeing fed through the printing nip 14.

It is easy to see that the requirement for the lips 16 to be parallel isgreater, the narrower the zone is in which the lip 16 and screen 23contact each other. It is consequently useful to select the elasticityof the material of the working doctor blades 19 and the pressure of thedoctor blade device 24 against the screen 23 to be great enough for asufficient width of the contact zone of the lips 16 to be produced. Withan axial length of the screen-printing cylinders 12 and 17 ofapproximately 800 to 900 mm, it is desirable for the contact zone toextend at least 0.05 to 1 mm, preferably approx. 0.3 to 0.4 mm in thecircumference direction of the screen-printing cylinders 12 and 17. Withaxial lengths that differ from this, a proportionally larger or smallerwidth of the contact zone can be selected.

In accordance with a second preferred embodiment of the presentinvention, as shown in FIG. 4, the doctor blade device 24 of thescreen-printing cylinder 17 remains in contact with the screen 23 evenduring its passage through the indented screen section 31. In this case,in order to prevent the doctor blade device 24 from deforming the screen23 in the screen section 31, the screen 23 is supported on the outsideof the screen 23, in the area of the screen section 31 by a supportelement 36. The support element 36 here has the shape of a basin or atrough that is uniformly curved in cross section. The support element 36may be, for example, made of sheet metal or a rigid plastic, and isanchored at its two axial ends to the ends of the screen-printingcylinder 17. Advantageously, the leading and trailing ends of the screen23 are also disposed in the screen section 31, where they are covered bythe support element 36. The support element 36 can be adjusted in theradial direction, which is indicated by the arrow 37 in FIG. 4, in orderto set the tension of the screen 23.

The screen-printing cylinder 12, which cooperates with theabove-described screen-printing cylinder 17 has a corresponding supportelement 36 in its indented screen section 31. This may also be seen byreferring to FIG. 4.

Since no ink can be transferred from the screen-printing cylinders 12;17 to the print stock in the indented screen sections 31 supported bythe support element 36, it is useful for the support element 36 to be aclosed plate which does not permit any ink 34 to pass through and reachthe exterior of the screen-printing cylinder 12; 17.

FIG. 5 depicts an alternative construction of the screen-printingcylinder 17 of the present invention.

The support element 36 depicted in FIG. 5 is affixed radially inside thescreen 23. The leading end 42 and the trailing end 43 of the screen 23overlap each other in the indented screen section 31 which is supportedby the support element 36. In this manner, the sensitive connectionbetween the two ends 42 and 43 of the screen 23, which two screen ends42 and 43 can be welded to each other, for example, is protected fromcontact with the lip 16 of the doctor blade device 24 and thus frompremature wear.

Also in this embodiment of a screen-printing cylinder 17, as shown inFIG. 5, an adjusting element can be provided for adjusting the doctorblade device 24 in the radial direction, for example in the guide slot26 shown in FIGS. 3a, 3 b. However, since in this embodiment, the innerradius of the surface of screen 23 that is wiped by the doctor bladedevice 24 only fluctuates minimally, it is sufficient to compensate forthese minimal fluctuations solely by the provision of a radiallyflexible support, which is not specifically shown, for the doctor bladedevice 24.

In order to prevent synchronization errors in the screen-printingcylinder 17 with such a construction, when the working doctor blade 19passes a leading edge 46 or a trailing edge 47 of the support element36, the subject invention provides that these edges 46 and 47 do notextend exactly parallel to a generatrix of the outer surface of thescreen-printing cylinder 17 or to the lip 16 of the doctor blade device24, but extend at a slight angle with respect to that generatrix or lip.For example, a sawtoothed, rafter-shaped, or sinusoidal curve of theedges 46 and 47 is possible. Preferably the edges 46 and 47 eachrepresent a helix with a pitch that is a multiple of the axial length ofthe screen-printing cylinder 17. When the doctor blade device 24 isrunning onto them or off of them, such a curvature of the edges 46 and47 prevents braking or acceleration forces acting on the screen-printingcylinder 17 from being exerted only at a certain point in time and at aparticular angular position of the screen-printing cylinder 17. Instead,these forces are distributed over a circumferential section of thescreen-printing cylinder 17 which, depending on the dimensions of thescreen-printing cylinder 17, can be from several millimeters up to a fewcentimeters wide. This smoothes the torque required to drive thescreen-printing cylinder 17 and prevents synchronization errors.

FIG. 6 is an axial section view in the vicinity of a printing nip 14 ofanother preferred embodiment of the printing unit 4 according to thepresent invention, in the same phase of its rotational movement as theone shown in FIG. 3b. Elements which are depicted in FIG. 6, andcorrespond to those that have already been described in relation toFIGS. 3a and 3 b, are provided with the same reference numerals in FIG.6 and will not be described again in the following discussion.

The difference between this depicted embodiment of the sheet-fedprinting press with screen-printing cylinders, in accordance with thepresent invention, and the ones described with reference to FIGS. 3a and3 b, 4, and 5 lies in the configuration of the doctor blade devices 24.The printing unit 4 depicted in FIG. 6 has two differently embodieddoctor blade devices 24 a and 24 b. Each of them has a working doctorblade 19 a, 19 b, respectively which is made of an elasticallydeformable material with a lip 16, which, during the course of therotary motion of the screen-printing cylinder 17, sweeps along theinside of the screen 23 in order to press ink 34 through screen 23.Separate from the lips 16 and parallel to them, each doctor blade device24 a and 24 b has a counter-pressure device 21 a or 21 b, respectivelyin the form of a bridge that extends, in the axial direction of thescreen-printing cylinder 17, over the same length as the working doctorblade 19 a or 19 b and which has an end surface 13 which sweeps alongthe inside of the screen 23. Each counter-pressure device 21 a or 21 bis respectively aligned so that an end surface 13 of each suchcounter-pressure device 21 a or 21 b is disposed opposite from the lip16 of the working doctor blade 19 b or 19 a of the respective opposingdoctor blade device 24 b, 24 a. The purpose of these counter-pressuredevices 21 a and 21 b is to provide a buttress or a counter-actingsupport for the pressure exerted by the opposing working doctor blade 19b or 19 a, respectively, which buttress or support assures that asufficient quantity of ink passes through the screen 23 and is printedwith high quality on print stock being fed through the printing nip 14.The end surface 13 of each of the counter-pressure devices 21 a and 21 bcan have a width of several millimeters in the circumference direction.This assures that, even when the two doctor blade devices 24 a and 24 bare slightly out of parallel, the pressure of each lip 16 is compensatedfor over its entire length by the cooperating counter-pressure device.As a result, a uniform printing quality is achieved over the entirewidth of the print stock.

In the doctor blade device 24 a, the working doctor blade 19 a isdisposed before the counter-pressure device 21 a in the rotationdirection of the screens 23. In the doctor blade device 24 b, this isreversed. The doctor blade device 24 b can therefore be embodied as achamber doctor blade in which the counter-pressure device 21 bconstitutes a closing doctor blade. The chamber defined by the workingdoctor blade 19 b and counter-pressure device 21 b can be subjected topressure in order to control the flow rate of the ink provided in thechamber defined by the working doctor blade 19 b and thecounter-pressure device 21 b.

Although not specifically shown in FIG. 6, in this embodiment of thescreen-printing cylinders 12 and 17, the indented screen sections 31 canbe equipped with support elements 36, as shown in FIGS. 4 and 5.

While preferred embodiments of a sheet-fed printing press withscreen-printing cylinders in accordance with the present invention havebeen set forth fully and completely hereinabove, it will be apparent toone of skill in the art that various changes in, for example the drivesfor the cylinders, the types of sheets being printed, and the like canbe made without departing from the true spirit and scope of the presentinvention which is to be limited only by the following claims.

What is claimed is:
 1. A printing unit comprising: a firstscreen-printing cylinder having a first doctor blade device including afirst working doctor blade and a first counter-pressure device, saidfirst screen-printing cylinder being rotatable in a first direction; asecond screen-printing cylinder having a second doctor blade deviceincluding a second working doctor blade and a second counter-pressuredevice, said second screen-printing cylinder being rotatable in a seconddirection, said first screen-printing cylinder and said secondscreen-printing cylinder being in direct contact, said counter-pressuredevice in each said first and second doctor blade devices beingpositioned in its respective one of said first and secondscreen-printing cylinders to support said working doctor blade in eachsaid second and first doctor blade devices in its respective one of saidsecond and first screen-printing cylinders; and ink disposed in saidfirst screen-printing cylinder between, in said first direction ofrotation, said first working doctor blade and said firstcounter-pressure device, and ink disposed in said second screen-printingcylinder before, in said second direction of rotation, said secondworking doctor blade and said second counter-pressure device.
 2. Theprinting unit of claim 1 wherein said first doctor blade device is achamber doctor blade and said counter-pressure device is a closingdoctor blade of said chamber doctor blade.